Welcome to the wonderful world of ATI!
I started a thread about a month ago about our problem.
http://forumz.tomshardware.com/hardware/modules.php?name=Forums&file=viewtopic&p=1451552#1451552
So far, I've tried: X1950 PRO 256/512 AGP Visiontek, Diamond, Sapphire, and the latest, HIM and nothing's changed, no signal output after 10 mins to start, but it decreases with each reboot, leading me to believe in the heat theory. Interestingly, it's fine in 2D, using the win drivers, but as soon as I load the ATI drivers it takes a dump.
Just for grins, I'm going to try it in another build this week and see what happens, (4 RMA's anyone?) in the meantime I'll suffer with my 7800GS. : (
3.2 P4/Asus P4V8X-MX/2GB Corsair XMS/750W Thermaltake....